Glycoconjugates and their antibodies are vital components of host-tumor interaction. This review concentrates on the oncological implications of research concerning the alpha gal triad; the alpha 1-->3 galactosyl epitope (alpha Gal), the enzyme responsible for its construction, alpha 1,3 galactosyl transferase (alpha 1-3GT), and its associated antibody: anti-gal. Alpha gal epitopes, previously assumed to be absent from human tissue, have been demonstrated on several human cancer cell lines, senescent red blood cells, and Graves' disease thyrocytes. Alpha-gal presence on neoplastic lines is correlated with increased metastatic formation in animal models. The mechanisms of human response to these neoantigens are complex, as natural anti-gal antibodies exist in high titers in normal sera, thus predicting immunological recognition of cells expressing alpha gal epitopes. Hypotheses vary regarding the pathogenic contributions of metastasis-associated phenomena such as de novo expression of alpha gal and its unmasking by desialylation. The means by which alpha gal is sporadically expressed in human tissue remain unknown, as the galactosyl transferase which produces this epitope in constitutively expressive animals has undergone significant mutation at the genomic level in humans. Pathological re-expression is presumed to require permissive changes at a cellular level. Detailing these alterations is a prerequisite to the comprehension of the metastatic phenotype. In this context, the possibility of therapeutic strategies affecting alpha gal expression are also discussed.
Christiane Y, Aghayan M, Emonard H, Lallemand A, Mahieu P, et al.
Placenta •
The galactose alpha 1-3 galactose (Gal alpha 1-3 Gal) residue is a carbohydrate widely distributed in many non-human mammals. Since Gal alpha 1-3 Gal residues are described on the cell surface of tumor cells, we have examined the possibility of their expression on human trophoblastic cells at different stages of placental implantation and in various pregnancy-associated conditions. Using immunohistochemical methods, Gal alpha 1-3 Gal was demonstrated on interstitial and vascular trophoblast during pregnancy. For villous trophoblast, the staining disappeared in second trimester pregnancies. The density of staining for Gal alpha 1-3 Gal was increased in highly invasive trophoblast (mole and choriocarcinoma) and decreased in poorly invasive specimens (spontaneous abortion, XO monosomia). No cells displaying Gal alpha 1-3 Gal at their surface were identified in some segments of spiral arteries from pre-eclamptic women. The anti-Gal antibody titer increased in the first trimester of pregnancy and in the sera of pre-eclamptic and eclamptic patients. These findings suggest that Gal alpha 1-3 Gal residues could be considered as markers for trophoblast invasive capacity and that the binding of maternal anti-Gal antibodies to the trophoblast could contribute to limit trophoblastic invasion and thus participate to the immunological control of implantation.
As much as 1% of circulating IgG in man (the natural anti-Gal antibody) is directed against the alpha-galactosyl epitope, with the structure Gal alpha 1----3Gal beta 1----4GlcNAc-R. The alpha-galactosyl epitope is abundantly expressed on cells of nonprimate mammals, prosimians, and New World monkeys. Its expression is diminished in Old World monkeys, apes, and humans. It has been previously suggested that interaction between anti-Gal and aberrantly expressed alpha-galactosyl epitopes on thyroid cells may contribute to the initiation of autoimmune thyroid disorders. To study this possibility, alpha-galactosyl epitope expression on thyroid cell membranes of normal individuals and patients with Graves' disease was assessed by a sensitive radioimmunoassay. alpha-Gal-actosyl epitopes were found both on normal and diseased thyroid cells. Whereas the concentration of these epitopes on Graves' disease thyroid membranes was somewhat higher than that observed in normal glands, the difference was not significant. The activity of the enzyme, alpha 1-3-galactosyltransferase, which synthesizes the alpha-galactosyl epitope, was higher in microsomal fractions obtained from some patients as compared with healthy controls, but not significantly different. In view of the abundance of anti-Gal antibody in the circulation, it is argued that, under physiologic conditions, the interaction of this antibody with alpha-galactosyl epitopes does not elicit pathologic effects. However, aberrant expression of the alpha-galactosyl epitope may result in effective anti-Gal binding to thyroid cells (e.g., rearrangement of this structure on the cell membrane or its increased expression).(ABSTRACT TRUNCATED AT 250 WORDS)
Castronovo V, Foidart JM, Li Vecchi M, Foidart JB, Bracke M, et al.
Invasion & metastasis •
The lung colonization of MO4 cells, a highly malignant murine cell line, is strongly reduced in syngeneic C3H/He mice by a prior incubation with anti-alpha-galactosyl antibody (alpha-Gal Ab), a natural IgG antibody present in high titers in all normal human sera and specifically recognizing Gal alpha(1----3) structures (alpha-D-galactopyranosyl; alpha-D-Galp). The protective effect is due to a binding of alpha-Gal Ab to alpha-D-Galp end groups of MO4 cells, inducing both an increase in their sequestration into the liver and the spleen and a decrease in their sequestration into the lung. The F(ab')2 fragments of this antibody also exhibit a protective effect by inhibiting the homing of MO4 cells into the lung, without modifying their accumulation into the spleen and the liver. Since both the antibody and the alpha-galactosidase pretreatments of MO4 cells block their subsequent attachment to murine laminin in vitro, we suggest that, in this model, the lung colonization may be dependent on the alpha-D-Galp end groups exposed on the surface of MO4 cells.
